This PR includes the patches and changes to Uenv that I have added so far in order to deploy a proof on concept on Los Alamos systems (as part of my summer intern project). Hopefully any feedback on this PR can be a starting point for eventually integrating our changes into the production Uenv tool, and should give you a sense of the features we would like to have in our environment. The summary is:

• Global configuration file (toml format; parsed using the toml++ library)
  • Currently defines any local default repos with system provided uenv, in addition to user repo
  • Boolean for using squashfuse (rootless) instead of squashfs-mount (setuid binary) for mounting uenv
  • Currently located by the UENV_CONFIGURATION_PATH env var (not a great solution).
• Added support for squashfuse option for uenv start and uenv run (this is dependent on the patched squashfs-mount tool we've been using; see PR on that repo)
• Added support for using local repos in uenv image ls and in the concretise_env() function so that uenv start and uenv run search them.

P.S. I am by no means a strong C/C++ programmer, so this code is probably missing some things such as edge case functionality and certain error handling. It should still give a good idea of where we're headed though :).